7v Fan Mod

I have done this mod and it seems to work fine. Maybe I need a lesson in basic electronics though.

If I send +12v down the +5v line, I get +7v. But where does it go? Would that make the entire +5v line +7v before it gets back to the psu? Would it be safe to attach drives on the same power chain?

Also, does the same idea apply to fans that attach to the mobo? And why do fans with mobo connectors have both a +12 and a +5?

Originally posted by WackDaddy


Also, does the same idea apply to fans that attach to the mobo? And why do fans with mobo connectors have both a +12 and a +5?

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the yellow/white wire on 3 pin motherboard conectors are the signal wires, i.e. the wire that send the signal to the motherboard telling it the RPM of the fan.

Originally posted by MAXXMODDER
the yellow/white wire on 3 pin motherboard conectors are the signal wires, i.e. the wire that send the signal to the motherboard telling it the RPM of the fan.

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Ah that makes sense, should have thought of that. Thanks. Anyone for the other part?

To get 7v fans, I read that you put the 12v to the fan's power wire and the 5v line to the fan's ground wire

It happens because voltage is the potential difference between two points. So if you hook the 5v line to the ground this makes the ground 5v instead of 0v. Hence sending the 12v through the power only creates a potential difference of 7v (12v comes in and the ground is at 5v hence 7v) So it isn't really going anywhere it is still there but the rules have changed slightly.

Originally posted by ChAotiCIllusion
It happens because voltage is the potential difference between two points. So if you hook the 5v line to the ground this makes the ground 5v instead of 0v. Hence sending the 12v through the power only creates a potential difference of 7v (12v comes in and the ground is at 5v hence 7v) So it isn't really going anywhere it is still there but the rules have changed slightly.

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Yes I got that. But what do you mean it isn't going anywhere? If the electricity wasn't going anywhere, the fan wouldn't be turning. Right? Or are you saying that because it is a potential difference that the psu is only putting out +7v there?

I guess what I'm getting at is, does that extra voltage effect other devices in the chain before the psu? Wouldn't it make all of the red and yellow wires +7v before it gets back to the psu?

i did this to an antec truepower once and it went boom. so be careful.

Voltage is all about differences in potential. Ground is 0, +5V is 5, +12V is 12 with ground being set to 0 and the rest relative to ground. Between the +5V and +12V rails there is a potential of 7 volt. Since there's no such thing as a 'true ground', any wire can function as the ground.

However, in switching PSUs, wiring stuff together which was never designed to be connected may do funny things with the voltage balancing circuitry of the PSU, causing it to output voltages below or above the 5 or 12 volts (+/- 10%), so it's not the safest thing to do.

Originally posted by WackDaddy
Yes I got that. But what do you mean it isn't going anywhere? If the electricity wasn't going anywhere, the fan wouldn't be turning. Right? Or are you saying that because it is a potential difference that the psu is only putting out +7v there?

I guess what I'm getting at is, does that extra voltage effect other devices in the chain before the psu? Wouldn't it make all of the red and yellow wires +7v before it gets back to the psu?

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Voltage is the potential between any two points. Hence to get 12v you conenct one wire to 12v and the other to the ground. the ground is at 0v giving you a difference of 12v-0v. However when you do the fan mod the ground is now at 5v. This means that the potential difference between the voltage in and the ground is only 7v. A power supply cannot actually put out 7v you just create the 7v potential difference by changing the ground from 0v to 5v hence the potential difference is no 5v less because the ground is 5v higher.
So to answer the question 7v is only given to the device you hook up in a way as to give it 7v. In a chain it doesn't make the slightest difference because the power supply can only output 12v and 5v.

Originally posted by ChAotiCIllusion

So to answer the question 7v is only given to the device you hook up in a way as to give it 7v. In a chain it doesn't make the slightest difference because the power supply can only output 12v and 5v.

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Like I said, I got what voltage is and I think you are even more confused than I am. It's not like one side of the molex connector is 7v and the other is 5v. If you make that connection then you are forcing voltage down the 'wrong' path.

So let me ask this now: Does this feedback present a problem for the PSU? I'm guessing a modern power supply can handle it. Also, does this potential undervolting and/or overvolting present a risk for any other devices?

the yellow/white wire on 3 pin motherboard conectors are the signal wires, i.e. the wire that send the signal to the motherboard telling it the RPM of the fan.

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that makes sense.. i always thought the red wire was positive, and the black wires negative and the yellow ones ground..lol... i am so stupid

Originally posted by WackDaddy
Like I said, I got what voltage is and I think you are even more confused than I am. It's not like one side of the molex connector is 7v and the other is 5v. If you make that connection then you are forcing voltage down the 'wrong' path.

So let me ask this now: Does this feedback present a problem for the PSU? I'm guessing a modern power supply can handle it. Also, does this potential undervolting and/or overvolting present a risk for any other devices?

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Voltage is simply the potential difference between two points, voltages do not move they induce a current to move, it is the push that forces the current to move. Current is the actual power in the circuit as this is related to the number of electrons that are in the circuit. Now voltage is an arbitrary value hence is changeable depending on what we can do. What the 7v mod does is to hook the psu's 12v to the fans 12v and the fans ground to the psu's 5v. This now means that the ground on the fan has become 5v. Due to the ground being 5v there is now only 7v of difference between the voltage coming in and the voltage at the end. Now by the law of V=IR we can see that a drop in voltage with the resitance being constant therefore the current must drop to keep the equation balanced. The big mistake lots of people make is to assume the power comes from the voltage when in fact it comes from the current.

As to the damage it shouldn't cause any unless you actually rewire the psu plug and it's the first on a chain and you plug something else into the last plugs on the chain as they will only get a weird supply of power. However if the rewiring is between the fan and the molex plug it should just lower the voltage going to the fan.

Originally posted by WackDaddy
Does this feedback present a problem for the PSU? I'm guessing a modern power supply can handle it. Also, does this potential undervolting and/or overvolting present a risk for any other devices?

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This will not affect the PSU at all, unless the PSU has circuitry to handle power changes (some of the higher antecs do...), the other devices will not be affected, as the voltage doesn't actually "go anywhere", due to the fact you're just swapping the ground wire for the +5V wire, only making +7V at the fan.

hope this helps, if you want, i'll dig up the actual theory behind it...heh

Originally posted by graphinesys
This will not affect the PSU at all, unless the PSU has circuitry to handle power changes (some of the higher antecs do...), the other devices will not be affected, as the voltage doesn't actually "go anywhere", due to the fact you're just swapping the ground wire for the +5V wire, only making +7V at the fan.

hope this helps, if you want, i'll dig up the actual theory behind it...heh

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Not that I don't trust you. And I'm not worried about killing anything since it seems to be working fine for most people. But as ChAotiCIllusion said: "Voltage is simply the potential difference between two points."

Now look at my hdd on the same chain as the mod.
This causes the +5 and +12 wires that are going into my hdd to be connected.

I decided to try a little test. I took a molex splitter and did the swap. I plugged my hdd into one of them and left the other empty. Then I plugged my fans into another molex connector on the chain. They spun at 7v. So by you theory, only the hdd should be getting 7v?

Originally posted by WackDaddy
Not that I don't trust you. And I'm not worried about killing anything since it seems to be working fine for most people. But as ChAotiCIllusion said: "Voltage is simply the potential difference between two points."

Now look at my hdd on the same chain as the mod.
This causes the +5 and +12 wires that are going into my hdd to be connected.

I decided to try a little test. I took a molex splitter and did the swap. I plugged my hdd into one of them and left the other empty. Then I plugged my fans into another molex connector on the chain. They spun at 7v. So by you theory, only the hdd should be getting 7v?

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If you did the mod on the fans molex the hard drive will be getting exactly what it needs with the 12v 5v and the 2 grounds in exactly the right place. If you did the mod on the psu molex for some werid reason then the hard drive would be getting a messed up supply because the hard drive needs both the 12v and 5v to run. If the mod is on the fans molex only it's potential difference will change. On the PSU molex it could change the values of the molexes also connected to it but it will change depending on how the devices are chained.

What it boils down to is only change the fans molex so you connect it's ground to 5v and you're safe.